Mutation of Critical Target Genes, e.g., the tumor suppressor gene p53, are an important step in the development of human cancers. Although the causes of these mutations are in many instances unknown, much evidence suggests that DNA damage induced by reactive oxygen species (ROS) may be an important source of promutagenic DNA damage in some human neoplasms, including breast cancer. We propose to investigate the 'oxidative DNA damage hypothesis' using the molecular epidemiology approach. Using H2O2- induced damage/repair distributions by LMPCR and the H2O2-induced mutational hotspots by constant denaturant capillary electrophoresis in a target gene of identical target cells, using several exons of the HPRT gene of cultured human TK6 cells as a selectable target gene. The data accrued from these studies may provide insight into the molecular mechanisms involved in breast cancer development by providing evidence supporting the preliminary indications that oxidative DNA damage plays a role in the neoplastic transformation of human breast epithelium.